Noting that many people were likely afraid to sleep indoors, citizen's protection minister Michalis Chrysohoidis said additional police would be deployed and remain on standby for the entire weekend.

"We are here next to you, we will not leave, we will not sleep until all danger has receded...we will overcome this together," Chrysohoidis said in a televised statement.

The quake struck at 1113 GMT northwest of Athens, close to where a 5.9-magnitude quake in September 1999 left 143 people dead in and around the capital.

State TV ERT reported that at least four people were slightly hurt -- including a pregnant tourist and a young boy -- and at least two abandoned buildings in the capital collapsed, while several more suffered damage.

Traffic jams were caused in parts of the capital where power cuts knocked out traffic lights.

"It was a very intense quake, we were terrified, everyone started coming out (of the building)," a young woman named Katerina told AFP outside the six-storey cosmetics store where she works.

A government spokesman later Friday said "life in Athens is back to normal".

Friday's emergency was the second in just over a week for the new conservative administration of Prime Minister Kyriakos Mitsotakis, who took office after a July 7 election.

In the city centre, the marble cupola cross of a historic 19th century church came loose and shattered onto the street below.

Another central Athens church had its cupola cross snapped by the tremor, and a disused cargo conveyor belt at the port of Piraeus collapsed, TV footage showed.

And a Byzantine-era monastery on the outskirts of Athens was also damaged, the culture ministry said.

- 'Like an explosion' -

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The culture ministry said the pregnant tourist was visiting the National Archaeological Museum and was struck by another visitor rushing for the exit.

She was hospitalised as a precaution, the ministry said.

The boy was struck by a falling chimney, ERT said.

News channels broadcast images sent in by viewers which also showed parked cars in central Athens damaged by fallen masonry.

Cracks also appeared in some walls in Greece's 170-year-old parliament building.

"It was more like an explosion," another woman told ERT on the city's central Syntagma Square.

"We were all very afraid, so we'll stay here for a couple of hours."

Government spokesman Stelios Petsas said on television that phone networks had become "overcharged" by the sheer number of users calling to check up on friends and family.

SKAI TV said there were 20,000 calls per second, more than on New Year's Eve.

The fire department rescued 30 people trapped in elevators in the capital, the citizen's protection minister said.

- 'Remain calm' -

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According to the Greek geodynamic institute, the quake had its epicentre was near the town of Magoula, 23 kilometres (14 miles) northwest of Athens, and was followed by over a dozen aftershocks.

It had a depth of 13 kilometres, it said.

State engineers were out checking buildings for structural damage, and two helicopters overflew the quake's epicentre, officials said.

Worried residents and office staff quickly gathered in outdoor areas, and department stores asked shoppers to leave but the government denied it had ordered an evacuation alert.

"People must remain calm," said Efthymios Lekkas, head of the state anti-quake protection agency.

"There is no reason for concern. The capital's buildings are built to withstand a much stronger earthquake," he told ERT.

The US geological institute said Friday's quake had a magnitude of 5.3.

"For the time being we cannot be sure whether this was the main earthquake," seismologist Gerassimos Papadopoulos told ERT.

"There have been at least three (smaller) aftershocks already, which is a positive sign," he said, adding that the quake was felt as far as the Peloponnese peninsula.

"People in the capital must remain calm... they must be psychologically ready for more aftershocks," he said.

- Major fault lines -

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In another natural disaster earlier this month, seven people died and at least 23 were injured -- most of them tourists -- as a fierce storm ripped through beachfronts in Halkidiki, one of Greece's top tourist areas on July 10.

Greece lies on major fault lines and is regularly hit by earthquakes, but they rarely cause casualties.

In July 2017, a 6.7-magnitude earthquake killed two people on the island of Kos in the Aegean sea, causing significant damage.